The present invention relates to nuclear magnetic resonance (NMR) imaging and nuclear magnetic resonance chemical shift spectroscopic imaging and, more particularly, to novel methods for overcoming transient inhomogeneities in the imaging magnetic field, especially as induced by the pulsed magnetic field gradients utilized in the imaging process itself.
It is now well known that nuclear magnetic resonance imaging can be utilized for in vivo studies, particularly of human patients, to image proton (.sup.1 H) densities and the like. It is also known to study other nuclear species in a heterogeneous sample by chemical shift spectroscopy and the like, at either a single site or at each of an array of a plurality of ordered sites in the sample. Of these studies, nuclear magnetic resonance chemical shift spectroscopy imposes the more demanding requirements upon a nuclear magnetic resonance imaging system. A pivotal requirement for the performance of magnetic resonance chemical shift spectroscopic imaging is that the magnetic field utilized must be sufficiently uniform so that a chemical shift spectrum is resolvable from each sensitive volume or image volume (voxel) element. For nuclear species, such as protons (.sup.1 H), phosphorous (.sup.31 P), carbon (.sup.13 C), and the like, to be studied by in vivo nuclear magnetic resonance spectroscopy, magnetic field homogeneities of better than about one part-per-million (ppm) across a voxel are necessary. This homogeneity requirement can extend to the entire imaged volume, in techniques such as the selective irradiation chemical shift imaging technique disclosed and claimed in my co-pending U.S. patent application Ser. No. 561,381, filed Dec. 14, 1983, and assigned to the assignee of the present invention, which co-pending application is included herein in its entirety by reference. Techniques such as the foregoing example require magnetic field homogeneity better than about 1 ppm. across the entire imaging volume.
In practice, many NMR imaging techniques, and particularly spectroscopic imaging techniques, employ pulsed magnetic field gradients. Examples of such techniques can be found in my U.S. Pat. No. 4,506,223 issued Mar. 19, 1985, and my U.S. Pat. No. 4,480,228 issued Oct. 30, 1984, both assigned to the assignee of the present invention and incorporated herein by reference in their entireties. Such magnetic field gradient pulses will often induce eddy currents in any conductor within a certain distance of the main magnetic-field-forming structure of the magnetic resonance system. In high-field systems utilizing superconducting magnets, the magnetic cryostat or other structural metal is within the gradient pulsed magnetic field and eddy currents will often be induced within these metal components. Each induced eddy current may decay at its own individual rate; each rate may be substantially slower than the decay rate of the pulse that generated that eddy current. As each eddy current may itself induce a transient magnetic field gradient which can persist after the original input magnetic field gradient pulse has subsided to an essentially zero magnitude, then transient magnetic field gradients can be generated which persist into the time interval when chemical shift information is to be acquired. Such persistent transient magnetic field gradients can destroy the ability of the system to acquire the proper response information. For example, in three-dimensional (3-D) or four-dimensional (4-D) transform (FT) spectroscopic imaging, as described and claimed in the aforementioned U.S. Pat. No. 4,506,223, response data must be acquired in the absence of magnetic field gradients; in selective irradiation imaging methods, such as disclosed and claimed in the aforementioned co-pending application Ser. No. 561,381, and now U.S. Pat. No. 4,585,993 the nuclear magnetic resonance response signal from the selected chemical species must be excited in the absence of such magnetic field gradients. It is, therefore, critical to the performance of chemical shift spectroscopy and chemical shift spectroscopic imaging, that induced transient magnetic field gradients be ameliorated to the greatest degree possible, if not completely prevented. It is also desirable, even in the somewhat more-tolerant conventional NMR experiment, to reduce induced gradient magnetic fields to a minimum, as a higher inherent field homogeneity permits reduced data acquisition bandwidths, thereby providing improvements in the signal-to-noise ratio, and prevents the normally non-spatially-selective radio-frequency (RF) pulse signals from becoming spatially selective in the presence of any transient magnetic field gradients.